A number of optical metrology devices now exist for characterizing the composition and structure of semiconductor wafers. Many of these systems include measurement beams which must be focused to very small spots to study very small features on the wafers. In order to maintain such small spots, an accurate autofocus system is required.
In the past, the optical metrology devices of the assignee included an autofocus system based on a Foucalt knife test principal. This mechanism is described in U.S. Pat. No. 5,978,074 incorporated by reference. This system included a rotating chopper wheel. It would be desirable to develop a system which did not require moving parts.
Other autofocus systems are described in U.S. Pat. Nos. 5,910,842 and 6,502,478,both incorporated herein by reference.
An autofocus system is disclosed for driving a sample into the focal plane of a primary microscope objective. A light source, preferably a laser, generates a collimated monitor beam which is focused onto the sample through the primary objective. The reflected beam passes back through the primary objective and is directed to a secondary focusing element. The beam is then split by a reflective wedge into two portions with different path lengths such that the beam waists of the two portions are spatially separated. An aperture is located between the two beam waists and equidistant therefrom when the sample is in focus. This location is also approximately where the two beams intersect.
A photodetector, preferably a bi-cell, is provided to measure the intensity of the two beam portions transmitted by the aperture. The measured intensity levels vary with respect to the position of the sample. By comparing the measured intensity levels, an indication of the position of the sample can be obtained. This indication can be used as a feedback signal to correct the position of the sample with respect to the primary objective.